The present invention concerns a method of calibration of a stereoscopic camera system. In particular, embodiments of the present invention concern a method of calibration of a stereoscopic camera system for use with a radio therapy treatment apparatus.
Radiotherapy consists of projecting, onto a predetermined region of a patient's body, a radiation beam so as to destroy or eliminate tumours existing therein. Such treatment is usually carried out periodically and repeatedly. At each medical intervention, the radiation source must be positioned with respect to the patient in order to irradiate the selected region with the highest possible accuracy to avoid radiating adjacent tissue on which radiation beams would be harmful.
A fundamental problem with radiotherapy is the need to position the patient in the same position, when obtaining diagnostic images and each of the subsequent times when radiation is applied to the patient's body. It is to that end that Vision RT have developed an image processing system for monitoring and positioning patients described in U.S. Pat. Nos.: 7,348,974, 7,889,906, 8,135,201, and pending U.S. patent application Ser. No. 12/379,108, published as U.S. 2009/0187112.
In use, in the Vision RT system, images of a patient on a mechanical couch are obtained by a set of stereoscopic cameras which are then processed to generate a 3D wire mesh model of the surface of a patient being monitored. This 3D wire mesh model is compared with a reference surface created during treatment planning. The relative positioning of the model and the reference surface is compared and used to generate instructions for the mechanical couch to position the couch, vertically, laterally and rotationally so as to match the surfaces and hence locate the patient reliably in the same location relative to the iso-centre of a treatment apparatus. Subsequently during treatment the position of a patient is continually monitored and if for any reason the patient moves or repositions themselves, this can be detected and appropriate action can be taken if necessary.
There are several sources of uncertainty in radio therapy treatment systems such as errors in patient positioning, target localization, and dose delivery. It is practically impossible to achieve perfect alignment mainly due to the presence of several geometric errors in the system. One of the critical geometric errors in radio therapy treatments is uncertainty in localizing the radiation field centre, which directly affects the dosimetric accuracy and results in incorrect tumour targeting that may lead to the delivery of inadequate dose to the lesion and/or serious damage to the healthy adjacent tissues. Therefore, it is necessary to develop methods to reduce the probability of such errors by extensive and efficient quality assurance programs to ensure high-level geometric accuracy of the treatment.
Originally, the primary method for iso-centre verification in radiotherapy was to measure the distance between the tip of a mechanical pointer mounted on the gantry head of a treatment apparatus and a fixed point mounted on the treatment table. Such a method was manual, laborious and time-consuming. The accuracy of the method depended upon the human observer and was also limited by size of the tip of pointer used.
An improved technique was introduced by Lutz, Winston and Maleki at Harvard Medical School in 1988 which is described in Lutz W, Winston K R, Maleki N. A system for stereotactic radiosurgery with a linear accelerator. Int J Radiat Oncol Biol Phys. 1988; 14(2):373-81. In the Winston-Lutz system a calibration phantom comprising a small metallic ball made of steel, titanium or tungsten is fixed on the treatment table by a locking mechanism. The phantom position is adjustable in three directions by means of a micrometer tool. The collimator used for radiotherapy is attached to the gantry head and the ball is placed as closely as possible to the iso-centre by aligning the marks on the phantom with the treatment room lasers. The collimated beam is used to expose a radiographic test film mounted perpendicular to the beam direction on a stand behind the ball. Differences between the centre of the sphere shadow and the field centre identifies the differences between the true iso-centre and the iso-centre as indicated by the treatment room lasers. The offset is read on each film using transparent template guidance scales or scanning the film and software analysis.
A mathematical method for analysing Winston Lutz images was developed and is described in Low D A, Li Z, Drzymala R E. Minimization of target positioning error in accelerator-based radiosurgery. Med Phys. 1995; 22(4):443-48 which used the film-measured iso-centre positional errors for eight gantry angle and couch settings to find the suitable offset for the phantom stand to minimize the distance between the treatment apparatus iso-centre and the target. A similar aim was followed by Grimm et al., who developed an algorithm to reconstruct the Winston-Lutz phantom ball locus in three dimensions from two-dimensional film images taken at certain couch and gantry angles and combined them with the images of lasers taken by digital cameras. This approach is described in Grimm J, Grimm S L, Das I J, et al. A quality assurance method with sub-millimetre accuracy for stereotactic linear accelerators. J Appl Clin Med Phys. 2011; 12(1):182-98.
A further example of automated processing of phantom images is described in E Schriebmann, E Elder and T Fox, Automatied Quality Assurance for Image-Guided Radiation Therapy, J Appl Clin Med Phys. 2009:10(1):71-79 which discusses the automation of Quality Assurance methods to ensure that a megavoltage (MV) treatment beam coincides with an integrated kilovoltage (kV) or volumetric cone beam CT. In the paper, a calibration cube is described as being located at the estimated location of treatment room iso-centre using laser markings. Images of the irradiation of the cube are then obtained and processed to determine the extent the cube as positioned is offset from the iso-centre as identified by the MV, kV and volumetric cone beams.
Calibration of stereoscopic camera systems for use in treatment rooms has developed alongside methods for identifying the iso-centre of a treatment apparatus. Calibration techniques used to calibrate the Vision RT patient monitoring system are described in U.S. Pat. Nos. 7,348,974 and 7,889,906.
As is described in U.S. Pat. Nos. 7,348,974 and 7,889,906 a calibration sheet comprising a 40*40 cm sheet of flat rigid material such as aluminium or steel on which a pattern revealing a 20*20 matrix of circles at known positions on the surface of the sheet is provided. Additionally, towards the centre of the calibration sheet are four smaller markers adjacent to four circles the centres of which together identify the four corners of a square of known size.
Images of the calibration sheet are obtained and processed to identify within the image the positions of the four markers in the images and their associated circles. From the relative positions of circles identified by the markers in the images, a projective transformation is determined which accounts for the estimated centres of the identified circles defining the corners of a parallelogram in the image which arises due to the relative orientation of the calibration sheet and the camera obtaining the image. The calculated transform is then applied to each of the identified circles in turn to transform the oval shapes of the circles. More accurate estimates of the positions of the centres of the four circles are then determined by identifying the centres of the transformed circles and utilising an inverse transform to determine the corresponding position of the estimated circle centre in the original image.
When the co-ordinates for all the centres of each of the representations of the circles on the calibration sheet have been calculated for an image, the relative orientation of the different cameras can then be calculated from the relative positions of these points in the images and the known relative locations of these circles on the surface of the calibration sheet as is described in detail in “A Versatile Camera Calibration Technique for High-Accuracy 3D Machine Vision Metrology Using Off the Shelf TV Cameras and Lenses”, Roger Tsai, IEEE Journal of Robotics and Automation, Vol. Ra-3, No. 4, August 1987. Further from the relative positions of the points in the individual images internal camera parameters such as the focal length and radial distortion within the camera images can also be determined.
Having determined the locations of the stereoscopic cameras, and any radial distortions present in the camera images, the positioning of the cameras relative to the iso-centre of the treatment apparatus is then determined. This is achieved by imaging a calibration cube of known size which is positioned on a treatment apparatus at a position with its centre at the iso-centre of the treatment apparatus as indicated by the co-incidence of marks on the exterior of the cube with the projection of the laser cross hairs which intersect at the iso-centre.
The images of the calibration cube are processed utilising the previously obtained measurements of the relative locations of the cameras and any data about the existence of any distortion present in the images to generate a 3D computer model of the surface of the cube. Since the cube has known dimensions and is at a known location and in a known orientation relative to the iso-centre of the treatment apparatus as indicated by the laser cross-hairs, a comparison between the generated 3D model and the known parameters for the size and position of the calibration cube enables measurements made in the co-ordinate system of the modelling software to be converted into real world measurements in the treatment room relative to the treatment iso-centre.
Although the conventional approach to calibrating a stereoscopic camera system for use with a radio therapy treatment apparatus is highly accurate, further improvements in accuracy are desirable.